We don't do this demo but it sounds interesting. A couple questions
come to mind. Are you sure the helium is at the stated temperatures when
you measure its volume? Also, is it possible you are trapping air under
the wrinkled balloon as you immerse it in LN?

Jerry

At 2/20/2002 01:45 PM, you wrote:

>Idea: Fill a balloon with helium, and insert it in a large pyrex beaker
>filled with liquid nitrogen. Measure the liquid N2 level before and after
>inserting the balloon to determine the displacement; this gives you the
>approximate volume of the deflated balloon. Insert balloon in another
>pyrex beaker of water at room temperature to determine the volume of the
>balloon. If PV=nRT applies, if He is a fairly good ideal gas, and if the
>pressure exerted by the nitrogen and water on the balloon is still
>approximately 1 atmosphere, the volume should decrease to 77/300 (26%) of
>its original volume; we measured 420/1170 (36%) the first time
>around. But our measurements are uncertain to within 100 ml, so that's
>420 plus/minus 100 ml, or about a 25% error. Now I need to work on a more
>precise measurement of the liquid nitrogen volume--maybe by weighing
>the beaker instead.
>
>Anybody else do this demo?
>
>Steve Wonnell